Representative among membrane separation processes for seawater desalination are reverse osmosis (RO), electrodialysis (ED), and forward osmosis (FO). To increase the longevity of the membrane and to maintain constant performance of membrane separation process, the scale that is formed as salt compounds are deposited on the membrane should be removed. Hence, a membrane separation process requires pretreatment of seawater to remove scale deposit.
Some seawater ingredients are deposited as scale on a membrane surface with the concentration and pH change thereof, thus degrading the performance of the membrane. Materials causative of scale deposition include calcium carbonate (CaCO3), calcium sulfate (CaSO4), magnesium carbonate (MgCO3), and magnesium sulfate (MgSO4). A desalination pretreatment process of seawater for eliminating such materials causative of scale deposition is dominantly carried out with chemicals or/and membranes (MF, UF). The chemicals for use in the pretreatment process include antiscalants for preventing scale deposition, or acids for maintaining the pH of seawater. Alternatively or in addition, membranes such as microfiltration membranes or ultrafiltration membranes may be used to remove ions causative of scale deposition in the pretreatment process. However, it is criticized that the load of removing scale deposits is just shifted from the desalination process to the pretreatment membrane process.
A bioelectrochemical system (BES) is designed to treat wastewater by use of electrochemically active bacteria, with the concomitant production of electrical energy or hydrogen, and examples of the bioelectrochemical system include a microbial fuel cell (MFC) and a microbial electrolysis cell (MEC). A microbial fuel cell is typically composed of two chambers, that is, an anode chamber and a cathode chamber, which are separated by an ion exchange membrane. A typical reaction that occurs on the anode with glucose as a substrate is shown in Chemical Formula 1. Under an aerobic condition, a reaction shown in Chemical Formula 2 typically proceeds on the cathode. In the anode chamber, biodegradable organic matter is treated to produce electrical energy while in the cathode chamber pH is continuously increased as OH− is generated.C6H12O6+6H2→6CO2+24H++24e−  [Chemical Formula 1]O2+4e−+2H2O→4OH−  [Chemical Equation 2]
On the other hand, a microbial electrolysis cell is structurally similar to a microbial fuel cell, but is designed to generate hydrogen from organic material by applying an electric current in the cathode chamber where an anaerobic condition is established. On the cathode, the solution continuously increases in pH as hydrogen gas is produced, as illustrated in the following Chemical Formula 3.2H2O+2e−→H2↑+2OH−  [Chemical Formula 3]
Abundant in ions, seawater has high electroconductivity and can be used as an electrolyte in an electrochemical system. When seawater is introduced into a cathode chamber of the bioelectrochemical system, such as microbial fuel cell and a microbial electrolysis cell, a reduction reaction occurs to generate electrical energy or hydrogen while hydroxide ions are produced to precipitate the polyvalent cations of seawater in the form of CaCO3, or Mg(OH)2. On the whole, bioelectrochemical systems are used for wastewater treatment, electricity generation, and hydrogen generation, but thus far have not been applied to the removal of polyvalent cations from seawater.